The effect of the coating process on the environment and the effect of the environment on coatings have increasingly shaped the coatings art in the last few decades. The industry has put considerable effort into developing coatings with materials that will be less harmful toward the environment, such as coatings containing lower levels of volatile organic compounds for a lower impact on the environment during their application. As another concern, it has been difficult to devise such a coating that will also have the desired resistance to environmental degradation.
One mode of environmental degradation that has attracted attention in recent years is environmental etch. "Environmental etch" is the name given to a kind of surface pitting and spotting that is thought to be due to the attack of environmental acids on the coating. Environmental etch manifests itself as spots or marks on or in the coating that often cannot be rubbed out. Environmental etch is particularly a problem where the coating is a high gloss and/or a high DOI (distinctness-of-image) coating, since defects in the surface of such a glossy or highly reflective coating are very noticeable. High gloss and/or high DOI coatings are widely utilized in the coatings art. The automotive industry has made extensive use of such coatings, often as color-plus-clear composite coatings for automotive body panels. Automotive coatings are especially likely to exhibit environmental etch because the significant outdoor exposure and the frequent heat buildup in the coated article are factors favoring etch degradation.
Prior art coating compositions such as high-solids enamels, although known for their durability and weatherability when used in exterior paints, have not provided the desired level of resistance to environmental etch when used as topcoat coatings. Environmental etch has been a particular concern with prior art high gloss or high DOI coatings. Compositions such as polyurethanes, acid-epoxy systems and the like have been proposed for use as topcoat coatings. However, many prior art systems suffer from disadvantages such as coatability problems, compatibility problems with underlying coatings, or high content of volatile organic compounds. Moreover, very few one-pack coating compositions have been found that provide satisfactory resistance to environmental etch, especially under the demanding conditions to which automotive coatings are subjected. Thus, there exists a continuing need for curable coating compositions with low content of volatile organic compounds that provide satisfactory resistance to environmental etch.